In general, there are three types of digital optical discs: read-only, recordable (also called Write-Once or Write-Once-Read-Many (WORM)) and rewritable (also called erasable). Examples of commercially available read-only optical disc technologies are the Compact Disc (CD) for digital audio and the Compact Disc-Read Only Memory (CD-ROM) for computer data. Compact Disc-Recordable (CD-R) drives and media are also commercially available. An example of a rewritable (erasable) optical disc technology is the Magneto-Optic (MO) disc, widely used for computer data storage. Rewritable compact disks (CD-RW), and rewritable Digital Video Disks (DVD, also called Digital Versatile Disks) will be commercially available soon.
Rewritable mass memory media is typically contained in a rigid cartridge during operation and the cartridge typically includes a write-protect feature to protect information on a storage medium from being destroyed. For example, most data tape cartridges have a moveable plastic tab that can be moved to one of two positions. Similarly, flexible diskettes in hard plastic cases have a sliding or breakaway plastic tab. CD's and DVD's, however, are not in a cartridge during operation. Write protection for CD's and DVD's must be implemented directly on the medium. Solutions have been proposed that are software based. That is, if a particular bit or data field has a particular value, software (operating system or drive controller) is not supposed to overwrite data on the medium. However, software solutions require support by all operating systems or drives. If a disk is created by one operating system or drive controller that supports a software write protect feature, and then is taken to a system that does not support the feature, data may be inadvertently destroyed. In addition, a software virus could bypass software write protection to delete data on the disk. There is a need for a physical write protect feature, directly on the disk medium, that protects against writing for any software.